Printing systems that employ upconversion phosphors that can efficiently upconvert near-infrared (NIR) light into visible and NIR luminescence using inexpensive diode lasers have numerous potential applications in science and technology. For example, such systems are useful in storage-disk production, solar cells, security applications, and transparent displays.
For security printing applications, most semi-overt/semi-covert and covert markings are printed with ultraviolet (UV)-to-visible inks. Yet, UV-to-visible downconversion security inks have become much easier to obtain and duplicate. Accordingly, printed features using these inks are easy to reproduce in the form of counterfeit items such as identity cards, currency, and other important documents. The use of upconversion security inks may provide the next generation of security. Kim et al. fabricated photopatterned microstructure films containing NaYF4 co-doped with rare earth ions for security type films that utilize NIR-to-visible upconversion. These photopatterned microstructures were developed to be excited by low powered, readily available, continuous wave diode lasers. Films were also printed in layers containing different ion compositions to generate various color-coded patterning. However, these technologies require a complex three-step process consisting of spin casting, photopatterning through a mask, and solvent-based pattern development.
The present invention utilizes an ink formulation in the form of a composite material, combined with a printing process capable of producing printed features ranging, for example, from 45 μm to 1 cm, from 20 μm to 5 cm, and/or from 10 μm to 10 cm. The prepared inks can be deposited via a number of different printing systems (including screen printing and aerosol jet printing) for either precision detailing or large scale production on a variety of surfaces. Printed features are generated in a single printing step, and little or no post-processing is required. These upconverting films are relatively high in sophistication and, as such, are useful for counterfeit protection.